if these eBooks have DRm and the two Kindles are not registered to the same abccount, the answer is no. The the eBooks are free of DRM, then the answer is yes. If the Kindles are registered to the same account then the answer is yes.

You can try it. Nothing is going to break. If the file is DRMd, it just won't open on the other Kindle. The book may be lendable, in which case you can lend it to your friend, even if it has DRM. Check the book's details on your Kindle, or on your Amazon.com Manage Your Kindle page.

if these eBooks have DRm and the two Kindles are not registered to the same abccount, the answer is no. The the eBooks are free of DRM, then the answer is yes. If the Kindles are registered to the same account then the answer is yes.

That's incorrect, Jon. Kindle DRM is device-specific. Even if both Kindles are registered to the same account, you still can't copy a book from one Kindle to another; you have to download it from Amazon to each device separately.

If you have bought the book you can loan it to another Kindle user I believe. You can look it up on your "manage your Kindle" and then click on the link to go to the page where the book is or go directly there if you remember the title. Then click on "loan book" at the top of the screen. Not sure of all the steps after that as I haven't ever loaned one but I believe they have the book as a loan to their account for 2 weeks.

Maybe I am missing something, but if the book is still free, your friend can simply download it. If it is no longer free, removing the DRM from the book you loaded. and then loading it to your friends device would be illegal, so we can not tell you how to do it (but google might)